Lithium secondary batteries have high voltage and high capacity relative to conventional nickel cadmium secondary batteries. In particular, when lithium transition metal composite oxides such as LiCoO2 and LiMn2O4 are used as cathode active materials, and carbonaceous materials such as graphite and carbon fiber are used as anode active materials, high voltage of greater than 4V and high capacity can be achieved, and side effects such as short circuits do not occur. Thus, lithium secondary batteries are widely used as power sources for mobile electronic devices such as cellular phones, notebook computers, digital cameras, etc.
Lithium secondary batteries are generally prepared by applying a slurry consisting of an active material and a binder on a metal film, drying the slurry and pressing the film. Although various resins have been used as the binder, fluorine-based resins such as polyvinylidene fluoride, which adheres well to the metal current collector and active material, is commonly used.
Nickel-based active materials have also been proposed as possible cathode active materials instead of the lithium cobalt-based active materials. Nickel-based active materials, generally represented by the formula LiNi1-x-yCoxMnyO2 (0≦x+y≦0.5, x>0, and y>0), have layered structures and high energy capacity of greater than 180 mAh/g. LiCoO2 has a capacity of about 145 mAh/g.
However, when mixing the active material with the fluorine-based binder to prepare the slurry, gelation can occur, resulting in increased viscosity and making it difficult to prepare a stable electrode. Increased slurry viscosity occurs due to the strong basicity of the slurry. The slurry is strongly basic because excess bases are used in preparing the nickel-based active material, which bases remain in the product. The basicity of the slurry causes hydrofluoric acid (HF) to separate from the fluorine-based resin due to double bonds which form between the bases and the fluorine resin. Activated oxygen or water molecules then bind to the double bonds, causing radical reactions. Finally, the crosslinking bonds produced by the radical reactions increase the molecular weight of the fluorine resin. The polyvinylidene fluoride should enhance adhesiveness to the substrate by increasing crystallinity, but the structure of the resin is weak to basicity. Thus, when the slurry is strongly basic and a lot of water remains in the slurry, the reaction rapidly progresses. Accordingly, there is a need to suppress the reaction.
Efforts to suppress gelation have included adding acids, etc. to the fluorine-based binder resin to neutralize the alkali contained in the cathode active material, thereby interrupting the effect of the alkali. Alternative efforts have including neutralizing the N-methyl-pyrrolidone solvent and the fluorine-based binder.
However, since the effect of suppressing gelation by the above methods is not significant, there is still a need for an improved method that can be applied in the practical process for preparing a battery by suppressing gelation substantially.